x . Badger et al . ( 2013 )
Fig . 10 . Boron ( panel a , blue points ) and alkenone ( panel b , red points ) pCO 2 reconstructions against carbon isotopes ( panel c ) from Badger et al . ( 2013 ) for the period 13.5-13.8 Ma . Red lines added .
This study employs both boron and alkenone proxies to a Miocene geological section in Malta ranging from 12.5 to 14 Ma . Most of the data are in the 250-350 ppm range , with the boron giving somewhat lower estimates than the alkenones . The data seem to show a declining trend .
4 . Synthesis
So let us return to the question : when was atmospheric pCO 2 last higher than 400 ppm ? All records agree it was before the Pleistocene (> 2.6 Ma ). The bulk of the data from the Pliocene epoch ( 2.6-5 Ma ), using a range of approaches and measurement techniques , are in the range 250-400 ppm and many records seem to indicate a declining trend through that time . We cannot conclude that pCO 2 was > 400 ppm in the Pliocene as a whole – if it was , it was only for short periods . Some of the alkenone